The present invention relates to a direct injection engine, particularly to a direct injection engine wherein fuel is directly injected into the combustion chamber for forming a tumble air flow.
According to the prior art, a direct injection engine is proposed wherein an injection nozzle of the fuel injector is provided in the combustion chamber of the engine, and fuel is injected for combustion from the fuel injector to the combustion chamber in the latter stage of the compression stroke in a light load/low speed range of the engine (Japanese Application Patent Laid-Open Publication NO. Hei 10-54246).
To put it more specifically, this direct injection engine is designed as follows: A fuel injector is provided on the peripheral area of the combustion chamber where swirl air flow is formed in a light load/low speed range. The distance from the injection nozzle of the fuel injector to the inner surface of the cylinder placed face-to-face with the injection nozzle is made longer than the distance for the arrival of fuel spray by injection during the time from the start of fuel injection to ignition. At the same time, the ignition plug is arranged so that the ignition gap of the ignition plug electrode is located in the fuel spray area injected from the fuel injector. The distance from the injection nozzle of the fuel injector to the ignition gap is made smaller than the distance for arrival of fuel spray.
In this direct injection engine, fuel is injected from the fuel injector into the combustion chamber where the swirl gas flow is formed, in the latter stage of the compression stroke in a light load/low speed range of the engine. As a result, stratified charge combustion is performed. At the time of ignition, spray fuel from the fuel injector does not reach the wall surface of the combustion chamber because of the configuration of the ignition plug, fuel injector and positional relationship between fuel spray angle and spray arrival distance. However, spray fuel is present around the ignition cap of the ignition plug. This prevents spray from being deposited on the wall surface of the combustion chamber, and ensures stable ignition, thereby allowing an effective stratified charge combustion to be performed.
In the direct injection engine of prior art, incidentally, stratified charge combustion is carried out by swirl air flow in a light load/low speed range, and the above-mentioned action takes place as a result. In a light load/high speed range, however, the piston traveling speed is increased by high speed rotation if swirl air flow remains in the combustion chamber. This makes it difficult to secure the time for spray fuel evaporation. As a result, the fuel injection time must be advanced. However, if the fuel injection time is advanced, fuel spray angle will increase due to low pressure in the combustion chamber, and spray fuel will be deposited on the inner surface of the cylinder head. This problem will cause another problem in stratified charge combustion operation in a light load/high speed range.
To solve such problems in the direct injection engine of prior art, stratified charge combustion is performed by swirl air flow in a light load/low speed range. Intake stroke injection is carried out in a heavy load/high speed range. At the same time, fuel is diffused through control of swirl ratio to provide uniform combustion. In a heavy load/high speed range, however, the sprayed fuel will be deposited on the top surface of the piston if fuel is diffused through control of swirl ratio to provide uniform combustion. This will make mixture with air difficult, and evaporation of spray fuel deposited thereon will be delayed. Fuel will tend to be discharged from the engine together with exhaust gas, without being burnt.
This results in increased amount of the unburnt hydrocarbon (THC) contained in exhaust gas discharged from the engine, causing environmental issues. At the same time, it will deteriorate engine performance and fuel economy.
The present invention has been made to solve these problems. Its object is to provide a direct injection engine capable of direct injection of fuel into the combustion chamber characterized in that discharge of gas containing hazardous gas component such as THC can be reduced and fuel economy can be improved by stratified charge combustion in the operation range of low to high speeds.
To achieve the above object, the direct injection engine according to the present invention is essentially characterized in that a tumble air flow is produced between the ignition plug located on the top of the combustion chamber and the fuel injector located on the side of the combustion chamber, and spray fuel is carried from the fuel injector to the ignition plug by this tumble air flow.
The direct injection engine as another embodiment of according to the present invention comprises an ignition plug arranged in the vertical axis direction of the cylinder, a fuel injector located on the axis line inclined with respect to t e horizontal axis perpendicular to the axis center of the cylinder, and an intake air control means. The intake air control means generates a tumble air flow in the combustion chamber, and the fuel injector discharges fuel from the intake air side in the combustion chamber toward the exhaust side. The ignition plug and the fuel injector are arranged in such a way that an angle xcex2 formed by a virtual straight line connecting between the ignition plug electrode and fuel injection point of the fuel injector and a horizontal axis line (X), and an spray top end angle xcex3 (elevation angle) formed between the spray outer edge of spray fuel and horizontal axis line are within the range xcex3=xcex2xc2x15 deg. As a fuel injector, the one equipped with a swirl-generating element upstream from the valve body is suitably used.
The penetration of spray fuel is preferred to be longer on the ignition plug side than on the piston side.
To achieve the above-mentioned object, the present invention is designed in such a way that the fuel spray injected from the fuel injector is carried to the ignition plug by the tumble air flow which reaches the plug after rising from below the fuel injector along the wall surface of the intake air side in the combustion chamber (called tumble guide method).
To put it more specifically, the fuel injector is designed to ensure that fuel is injected 3.15 msec. before ignition timing of the ignition plug. The fuel injector is also configured so that fuel is injected at 80 deg. before top dead point when mean effective pressure in the combustion chamber is 350 KPa at the engine speed of 3200 rpm.
The direct injection engine according to the present invention designed to have the above-mentioned configuration reduces the amount of fuel deposited on the top surface of the piston and inner wall of the cylinder block, and improves ignition property of the ignition plug.
In other words, spray fuel is carried by tumble air flow over a short distance from the fuel injector to the ignition plug on the side of the cylinder. This reduces the amount of spray fuel deposited on the top surface of the piston and inner wall of the cylinder block. It also increases spray fuel density close to the ignition plug, thereby improving ignition property by the ignition plug.
As a result, the direct injection engine according to the present invention allows stratified operations to be performed over an extensive range from the idling range to the high speed range. It also reduces the amount of spray fuel deposited on the upper surface of the piston and inner wall of the cylinder block. This, in turn, reduces the amount of THC contained in exhaust gas, improves the purification rate, and improves fuel economy.